Help needed for beer light circuit please

[krug0]

I have a beer light that uses a 12v 5w bulb. I have an old charger that outputs at 18vdc 400mA. I have cut the head off the charger and made a simple circuit with the bulb.
It obviously works but the voltage is too high for the bulb. I bought a volt meter today and the voltage in the circuit is actually about 25v. I bought a 10 ohm resistor from maplin
http://www.maplin.co.uk/10k-ohm-to-30-o ... ors-341625

When i attach it to the circuit, (before the bulb), and switch on, the bulb doesn't glow. But my volt meter reads a voltage of about 24.5v.

I thought my 10ohm resistor would reduce my (anticipated) 18v voltage down to about 12v but none of that has worked and i can't understand why the bulb won't glow?

Any thoughts? I presume i'm doing something very wrong.

Any help would be greatly appreciated.

Cheers

[I_Daniel]

At 12 Volt, 5 Watt the current draw is about 0.42 amp.

You must drop 13 Volt across the resistor. So 13 divided by 0.42 is about 31 Ohm. The nearest standard value is 33 Ohm.
13 times 0.42 is about 5.5 Watt so the resistor should be a standard 33 Ohm rated at 10 Watt.

Are you sure you are using a 10 Ohm and not a 10K Ohm resistor ?

Of course you can always switch to a 24 Volt 5 Watt globe or two 5 Watt 12 Volt globes in series. This leaves the Voltage at 25 Volt.

Remember after rectification and a smoothing capacitor the DC output voltage is 1.414 higher than the rated AC voltage.
In this case 18 times 1.414 equals 25.452 Volt. (You get the peaks !)

[krug0]

Thank you so much for your reply. Very helpful. Yeah i think i am using a 10k ohm, so that explains the problem i was having.

Things make a bit more sense in my head now, i'm on a bit of a learning curve at the moment, but you explained things brilliantly.

Thanks again